net: openthread: Fix for OT prompt in the Zephyr shell

OpenThread has changed it's behaviour in terms of prompt printing in the
CLI module. Previously it was only printed on the UART CLI backend, now
it's printed on every CLI backend. This results in a double prompt being
printed when combined with Zephyr shell (one from OT and other form
Zephyr).

This commit adds a temporary fix to prevent OT prompt from being printed
in Zehpyr shell. As a long term solution we should add an option to
OpenThread to allow to disable prompt on the output.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>